The present invention relates to compact, efficient transformer secondaries, and more particularly, to compact, efficient transformer secondaries having substantially optimized windings geometries in which the windings are indexed respective to adjacent windings.
Design of compact efficient transformer secondaries requires optimized usage of the area inside the magnetic path leading to minimization of coil resistances under the resulting, transient current conditions. In high voltage switch mode transformers, for example, the need to avoid excessive parasitic capacitance and large voltage output on each secondary may lead to a transformer design with a multitude of lower voltage secondaries whose outputs are series-connected to obtain the requisite high voltage.
Conventional transformer secondaries generally comprise a core material capable of containing magnetic flux, such as a soft iron or other similar material, a primary winding and secondary winding, each of which is disposed over the core material. These coils are generally constructed with the secondary winding formed by wrapping successive helical layers of an electrical conductor over the core material or other forming structure until the desired number of turns is established. Typically, each helical layer of such a construction will consist of several turns of the electrical conductor laid side by side extending longitudinally along the core material with the next layer beginning at the opposite end and traveling longitudinally back over the first layer. Such prior art is exemplified in FIG. 1. The electrical conductor normally used is commonly referred to as magnet wire and is a copper wire generally insulated with a coating of enamel or other like material thereon. In operation, each turn of the secondary coil winding will have induced in it a voltage produced by the changing magnetic field which links that turn and which is generated by changes in the current flowing in the primary winding. This magnetic field will induce approximately an equal amount of voltage in each successive turn of the winding, but as the individual turns are all serially connected, the voltage of each turn will be added to that induced in each preceding turn. Thus, it becomes apparent that while the turn-to-turn voltage gradient within the coil may be small, as the total number of turns within each layer increases, the layer-to-layer voltage gradient, being composed of the sum of the turn-to-turn voltage gradients within each layer of two adjacent radially disposed layers, will be of a considerable magnitude. This is particularly true when successive layers are wound with alternating longitudinal travel, that is, the first layer is wound with successive turns traveling from right to left with the next layer having successive turns traveling longitudinally from left to right. In this construction, the layer-to-layer voltage at the beginning end of the winding will be the sum of the turn-to-turn gradients for two complete layers of winding.
Although a multiple secondary approach can address excessive parasitic capacitance and large output voltage on each secondary, such designs using conventional wire and PCB coil forming techniques may result in physically large assemblies of fixturing for the many winding layers, starts, finishes, and layer transitions. This is especially true in high voltage power supplies above 30 kV where the designer may be interested in minimizing corona inception and thus may chose to use individual secondary voltages below 1 kV.
Low profile electronic components exist in the prior art, but most low profile designs are centered around “planar” designs formed from alternate layers of insulating material and copper foil or techniques involving coils formed on multiple layers of printed circuit board materials. These prior art designs, some of which are described above, involve a high cost and also have production disadvantages. Furthermore, typical printed circuit board insulators are considered inferior to those available on insulted winding wires.
Thus, what is desired is an optimized winding geometry which can be fixtured for compact implementation of a multitude of separate windings coupled to a common magnetic circuit. Such a desired winding geometry may include an index between adjacent layers where a conductor from one end of the coil may cross the adjacent turns and meet the conductor existing at the end of the turn at the opposite end of the layer, thereby substantially decreasing dimensional stack up of subsequent layers of such windings.